U.S. patent number 4,485,346 [Application Number 06/398,507] was granted by the patent office on 1984-11-27 for variable-energy drift-tube linear accelerator.
This patent grant is currently assigned to The United States of America as represented by the United States. Invention is credited to Thomas J. Boyd, Jr., James M. Potter, James E. Stovall, Donald A. Swenson.
United States Patent |
4,485,346 |
Swenson , et al. |
November 27, 1984 |
Variable-energy drift-tube linear accelerator
Abstract
A linear accelerator system includes a plurality of post-coupled
drift-tubes wherein each post coupler is bistably positionable to
either of two positions which result in different field
distributions. With binary control over a plurality of post
couplers, a significant accumlative effect in the resulting field
distribution is achieved yielding a variable-energy drift-tube
linear accelerator.
Inventors: |
Swenson; Donald A. (Los Alamos,
NM), Boyd, Jr.; Thomas J. (Los Alamos, NM), Potter; James
M. (Los Alamos, NM), Stovall; James E. (Los Alamos,
NM) |
Assignee: |
The United States of America as
represented by the United States (Washington, DC)
|
Family
ID: |
23575641 |
Appl.
No.: |
06/398,507 |
Filed: |
July 15, 1982 |
Current U.S.
Class: |
315/505;
313/359.1; 315/5.41 |
Current CPC
Class: |
H05H
9/00 (20130101); H05H 7/12 (20130101) |
Current International
Class: |
H05H
7/12 (20060101); H05H 7/00 (20060101); H05H
9/00 (20060101); H05H 009/02 (); H05H 007/12 () |
Field of
Search: |
;313/359.1,360.1
;328/233 ;250/492.2,492.3 ;315/5.41 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Swenson et al., "Variable-Energy Drift Tube Liniacs", Los Alamos
National Laboratory LA-UR-81-3696, Dec. 18, 1981. .
Swenson et al., "Variable-Energy Drift Tube Liniacs", Proceedings
of the 1981 Linear Accelerator Conference, LA-9234-C Feb., 1982,
pp. 187-190. .
Swenson et al., "Stabilization of the Drift Tube Liniac by
Operation in the .pi./2 Cavity Mode", 6th Int. Conf. on High Energy
Accelerators, CEAL-2000, 1967, pp. 167-173. .
Ghiorso et al., "Partial Energy Beams from an Ion Liniac", Proc.
1966 Linear Accelerator Conf., Dec., 1966, pp. 72-77..
|
Primary Examiner: Moore; David K.
Assistant Examiner: Wieder; K.
Attorney, Agent or Firm: Brenner; Leonard C. Gaetjens; Paul
D. Esposito; Michael F.
Government Interests
This invention is the result of a contract with the Department of
Energy (Contract No. W-7405-ENG-36).
Claims
What is claimed is:
1. A variable-energy linear accelerator system comprising;
a linear accelerator having an input for receiving a particle beam
at a particular energy level and an output for emitting said
particle beam at a higher energy level;
a plurality of drift-tubes forming a chain thereof between said
input and said output of said linear accelerator;
rf source means coupled to said linear accelerator for providing an
rf field therein for accelerating said particle beam through said
plurality of drift-tubes;
a plurality of post couplers within said linear accelerator, each
post coupler thereof individually associated with an individual
drift-tube in said plurality thereof, each post coupler thereof
being positionable to a first and second position, said first
position being an rf field unperturbing position and said second
position being an rf field perturbing position;
controllable positioning means coupled to each post coupler in said
plurality thereof for positioning each post coupler selectively to
said first and said second positions; and
control means coupled to said positioning means for controlling the
position of each post coupler in said plurality thereof.
2. The invention according to claim 1 wherein said second position
of each post coupler is a slight rf field perturbing position.
3. The invention according to claim 2 wherein each said slight rf
field perturbing position produces an rf field perturbation between
2% and 6%.
4. The invention according to claim 1 wherein said controllable
position means includes a plurality of binary positioning devices,
each binary positioning device therein individually associated with
an individual post coupler in said plurality thereof.
5. The invention according to claim 3 wherein each binary position
device in said plurality thereof includes an electrical solenoid
actuating unit.
6. The invention according to claim 3 wherein each binary position
device in said plurality thereof includes an air cylinder actuating
unit.
7. The invention according to claim 3 wherein said controls means
provides a separate binary control signal to each binary position
device in said plurality thereof.
Description
BACKGROUND OF THE INVENTION
The present invention relates generally to drift-tube linear
accelerators and more particularly to variable-energy drift-tube
linear accelerators.
Practical applications of proton and ion drift-tube linear
accelerators are more viable now than ever before because of the
development of the radio-frequency quadrupole (RFQ) accelerating
structure and other technological advances. Although many of these
applications would benefit from a variable energy option,
drift-tube linear accelerators are not noted for this property.
The only variable-energy method known to be in routine use involves
turning off later portions of the linear accelerator to provide a
few discrete energies from multitank linacs. Many applications
require more discrete energies than normally are available from
this scheme. Further, single tank, post-coupled drift-tube linear
accelerators are advocated for simplicity and reliability and any
multitank arrangement to provide energy variability represents a
step backward in linac technology.
Post couplers have a special property in that they can introduce a
step in the electric fields. Modest perturbations to the symmetry
of the post-coupler/drift-tube geometry can introduce few percent
cell-to-cell changes in the fields across the post coupler. Several
such perturbations on adjacent post couplers can introduce a
sizable reduction in the fields over the region of a few cells.
Such steps in the fields can be used to drop the beam out of
synchronism with the accelerating fields and provide a
variable-energy capability for the single-tank, post-coupled
drift-tube linear accelerator.
It is therefore an object of the present invention to provide an
improved drift-tube linear accelerator with a variable-energy
capability.
It is another object of the present invention to provide a reliable
post-coupler field-perturbation variable-energy drift-tube linear
accelerator.
SUMMARY OF THE INVENTION
To achieve the foregoing and other objects, and in accordance with
the purposes of the present invention, as embodied and broadly
described herein, the apparatus of this invention may comprise a
drift-tube linear accelerator having a plurality of post-couplers
wherein each individual post coupler thereof is positionable to a
selected one of two positions under the control of a post-coupler
controller. In the first position, called the home position, the rf
field is unperturbed and the post coupler forces a uniform field
distribution across the post coupler. In the second position,
called the alternate position, a preset perturbation causes a fixed
degree of asymmetry in the post-coupler/drift-tube geometry. In
this position, the post coupler introduces a small step of
prescribed magnitude of the field distribution across the post
coupler. By simple binary control of a plurality of post couplers,
a significant accumulative effect is achieved yielding a viable
variable energy drift-tube linear accelerator.
One advantage of the present invention is the provision of a
single-tank variable energy linear accelerator.
Another advantage of the present invention is that variable energy
in a post-coupled drift-tube linear accelerator is achieved under
simple binary control.
Still another advantage of the present invention is that variable
energy is obtained while each post coupler need be settable to only
two positions thereby simplifiying control, operation, reliability,
rf integrity, and vacuum integrity constraints.
Additional objects, advantages and novel features of the invention
will be set forth in part in the description which follows, and in
part will become apparent to those skilled in the art upon
examination of the following or may be learned by practice of the
invention. The objects and advantages of the invention may be
realized and attained by means of the instrumentalities and
combinations particularly pointed out in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and form a
part of the specification, illustrate an embodiment of the present
invention in a 100 cell drift-tube linac and, together with the
description, serve to explain the principles of the invention. In
the drawings:
FIG. 1 is an illustration of a variable-energy post-coupled,
drift-tube linear accelerator system in accord with the present
invention;
FIG. 2 is a diagram of a binary positionable post coupler used in
the linear accelerator system of FIG. 1;
FIG. 3 is a diagram of energy distribution in a linear accelerator
having ten 2%, 3%, 4%, 5%, and 6% post-coupler perturbations;
FIG. 4 is a diagram of energy distribution in a linear accelerator
having five, ten, fifteen, and twenty 4% post-coupler
perturbations; and
FIG. 5 is a diagram of energy distribution in a linear accelerator
having ten 4% perturbations beginning at every other post coupler
from number 50 through number 60.
DESCRIPTION OF THE PREFERRED EMBODIMENT
With reference to FIG. 1, the present invention, a drift-tube
linear accelerator 11, comprises a cylinderical cavity loaded with
a chain of drift-tubes 13a-13m, each drift-tube 13 therein commonly
having an associated post coupler 15 excepting possibly the first
drift-tube 13a and the last drift-tube 13m. It should be
appreciated that while for clarity purposes only a relatively few
drift-tubes 13 are shown in the drift-tube linear accelerator 11 of
FIG. 1, the invention certainly applies also both to larger and
smaller accelerators having more or fewer drift-tubes 13 in a
chain.
In operation, an injector 17 produces a low energy particle beam 19
and injects same into the drift-tube linear accelerator 11 which
accelerates the particle beam 19 with radio frequency electric
fields as the particles pass between the drift-tubes 13. As shown
in FIG. 1, the drift-tubes 13 get successively longer as the
particle beam 19 gains speed. The radio frequency fields result
from the introduction of rf power from the rf power source 21
through an rf power transmission line 23 into the drift-tube linear
accelerator 11. The distribution of fields within the drift-tube
linear accelerator 11 are stabilized and controlled by the post
couplers 15 located near the drift-tubes 13. Each post coupler 15
has two possible positions, namely a first position called a home
position as shown for post couplers 15b-15j, and a second position
called an alternate position as shown for post couplers 15k and
15l.
The position of each post coupler 15 is controlled by the
post-coupler controller 25. The accelerated particle beam 27
energing from the drift-tube linear accelerator 11 is directed
towards a target 28. The energy of the accelerated particle beam 27
depends on the distribution of the fields within the drift-tube
linear accelerator 11, which depends on the orientation of the post
couplers 15 that are controlled by individual post-coupler
positioner 29 that in turn are controlled by the post-coupler
controller 25.
The post-coupler controller 25 provides a signal to a post-coupler
positioner 29 associated with each post-coupler 15. The
post-coupler positioner 29 is capable of setting the associated
post-coupler 15 in either its home or alternate position. Each
post-coupler positioner 29 is individually associated with the
post-coupler controller 25 via signal control line 31.
With reference now to FIG. 2 may be seen how an individually
post-coupler positioner 29 controls the position of a post-coupler
15. The post-coupler 15 is pivotally secured to the post-coupler
positioner 29 within a housing 33. The housing 33 is secured to the
wall 35 of the drift-tube linear accelerator 11 and provides both
an rf and vacuum shield thereto. Although not shown in FIG. 2,
conventional engineering practices to achieve rf and vacuum
integrity would normally be employed such as metallic bellows, and
O-rings where appropriate. The post-coupler positioner 29 is
controllable to two positions for the post-coupler 15. Thus the
post-coupler 15 may be set to either its home or alternate
position. The post-coupler positioner 29 may be, for example, an
air cylinder having a spring return wherein the signal control line
31 would then be a tube for controlling air pressure.
Alternatively, the post-coupler positioner 29 may be an electrical
solenoid having two positions and the signal control line 31 then
would be an electrical line for carrying the electrical signal
designating which position the solenoid should assume. Ideally, the
home position of the post coupler 15 is the optimum energy position
wherein the rf field is unperturbed while the alternative position
of post coupler 15 provides an rf field perturbation resulting in
an energy decrease from the home position on the order of 2% to 10%
decrease. The specific amount of each perturbation is determined,
by presetting the amount of deviation between the normal and
alternate positions of the post-coupler 15. It is desirable to
operate near optimum position and therefore induce only a slight
perturbation. However, since a drift-tube linear accelerator 11 may
include a rather high plurality of drift-tubes 13, very slight
perturbations over a large number of drift-tubes 13 can generate a
very sizeable overall energy perturbation for the linear
accelerator 11.
Thus post couplers 15 fabricated in accord with the subject
invention as above described have a special property in that they
can introduce a step in the electric fields. Modest perturbations
to the symmetry of the post coupler/drift-tube geometry as
described can introduce a few percent cell-to-cell changes in the
fields across the post coupler 15. Several such perturbations on
adjacent post couplers 15 can introduce a sizable reduction in the
fields over the region of a few cells. Such steps in the fields can
be used to drop the particle beam out of precise synchronism of the
accelerating fields and thus provide a variable energy capability
for a single-tank, post-coupled drift-tube linear accelerator
11.
FIG. 3 illustrates the field distributions that can be established
in 100 cell, post-coupled drift-tube linear accelerator 11. FIG. 3
illustrates specifically the field distributions that result when
10 adjacent post couplers beginning at cell number 50 are set for
perturbations of from 2%, 3%, 4%, 5%, and 6%.
FIG. 4 shows the field distributions that result when 5, 10, 15,
and 20 post couplers are set for 4% perturbations, beginning at
cell 50.
FIG. 5 shows the result in field distributions when 10 post
couplers are set for 4% perturbations beginning at cells 50, 52,
56, 58, and 60.
Table I gives the field reduction factors for all combinations of
5, 10, 15, and 20 post couplers 15 set for perturbations from 2% to
10%. In all cases where the total perturbation is large enough to
drop the fields in the high-energy end of the drift-tube linear
accelerator 11 below the level required for synchronous
acceleration, the accelerated particle beam 27 will exit the
drift-tube linear accelerator at a reduced energy with some energy
spread. The resulting energies and energy spreads for a range of
perturbations near the center of a typical 100 cell 70 MeV
drift-tube accelerator 11 are given in Tables II-VI.
TABLE I ______________________________________ FIELD-REDUCTION
FACTORS FOR SOME COMBINATIONS OF THE NUMBER AND SIZE OF THE
INDIVIDUAL POST-COUPLER PERTURBATIONS Step Number of Steps Size 5
10 15 20 ______________________________________ 2% 0.98 0.9039
0.8171 0.7386 0.6676 3% 0.97 0.8587 0.7374 0.6333 0.5438 4% 0.96
0.8154 0.6648 0.5421 0.4420 5% 0.95 0.7738 0.5987 0.4663 0.3585 6%
0.94 0.7339 0.5386 0.3953 0.2901 7% 0.93 0.6957 0.4840 0.3367
0.2342 8% 0.92 0.6591 0.4344 0.2863 0.1887 9% 0.91 0.6240 0.3894
0.2430 0.1516 10% 0.90 0.5905 0.3487 0.2059 0.1216
______________________________________
TABLE II ______________________________________ AVERAGE ENERGY AND
ENERGY SPREAD IN MeV AS A FUNCTION OF THE NUMBER AND ORIGIN OF 2%
FIELD STEPS Origin of Number of 2% Steps Perturbations 5 10 15 20
______________________________________ 50 39.1.+-. 69.7.+-.
42.6.+-. 39.8.+-. 3.1 2.3 1.3 1.1 51 69.0.+-. 43.5.+-. 40.8.+-.
40.4.+-. 4.4 2.2 1.3 1.1 52 70.3.+-. 44.3.+-. 41.4.+-. 41.1.+-. 2.4
2.4 1.3 1.1 53 70.3.+-. 45.5.+-. 42.7.+-. 41.9.+-. 1.4 2.4 1.3 0.9
54 70.6.+-. 46.6.+-. 43.6.+-. 43.2.+-. 0.4 2.1 1.4 1.1 55 70.5.+-.
47.2.+-. 44.2.+-. 43.9.+-. 0.7 2.0 1.2 1.3 56 70.3.+-. 48.3.+-.
45.4.+-. 44.6.+-. 0.9 2.5 1.3 0.9 57 70.1.+-. 50.0.+-. 46.7.+-.
46.1.+-. 3.0 2.5 1.6 1.1 58 70.7.+-. 51.1.+-. 47.4.+-. 46.9.+-. 0.2
2.6 1.6 1.6 59 70.7.+-. 52.0.+-. 48.6.+-. 48.1.+-. 0.3 2.7 1.0 1.1
______________________________________
TABLE III ______________________________________ AVERAGE ENERGY AND
ENERGY SPREAD IN MeV AS A FUNCTION OF THE NUMBER AND ORIGIN OF THE
3% FIELD STEPS Origin of Number of 3% Steps Perturbations 5 10 15
20 ______________________________________ 50 46.7.+-. 36.9.+-.
36.2.+-. 36.2.+-. 4.7 1.0 0.6 0.6 51 47.8.+-. 38.1.+-. 36.9.+-.
36.9.+-. 4.9 1.2 0.9 0.8 52 48.4.+-. 38.8.+-. 37.9.+-. 37.9.+-. 4.6
1.1 0.8 0.6 53 50.1.+-. 40.0.+-. 38.7.+-. 38.8.+-. 5.8 1.2 1.0 0.9
54 50.8.+-. 40.8.+-. 39.7.+-. 39.7.+-. 5.0 1.2 0.6 0.6 55 52.1.+-.
41.5.+-. 40.5.+-. 40.6.+-. 4.9 1.1 1.0 0.9 56 53.0.+-. 42.9.+-.
41.4.+-. 41.4.+-. 5.6 1.2 0.9 0.8 57 54.1.+-. 43.7.+-. 42.4.+-.
42.3.+-. 5.8 1.4 0.6 0.5 58 56.6.+-. 44.6.+-. 43.5.+-. 43.4.+-. 6.4
1.1 1.2 1.0 59 57.8.+-. 45.9.+-. 44.4.+-. 44.3.+-. 6.3 1.3 1.0 0.9
______________________________________
TABLE IV ______________________________________ AVERAGE ENERGY AND
ENERGY SPREAD IN MeV AS A FUNCTION OF THE NUMBER AND ORIGIN OF 4%
FIELD STEPS Origin of Number of 4% Steps Perturbations 5 10 15 20
______________________________________ 50 39.1.+-. 34.8.+-.
34.2.+-. 34.4.+-. 1.9 0.8 0.5 0.4 51 40.3.+-. 35.4.+-. 35.1.+-.
35.2.+-. 2.1 0.7 0.7 0.7 52 41.2.+-. 36.4.+-. 35.9.+-. 36.1.+-. 2.1
0.9 0.6 0.6 53 42.2.+-. 37.2.+-. 36.8.+-. 36.8.+-. 2.1 0.9 0.8 0.8
54 43.7.+-. 38.4.+-. 37.8.+-. 38.0.+-. 2.0 0.8 0.5 0.5 55 44.2.+-.
39.1.+-. 38.6.+-. 38.7.+-. 2.1 0.8 0.8 0.8 56 45.1.+-. 40.1.+-.
39.4.+-. 39.6.+-. 2.2 0.8 0.5 0.5 57 46.6.+-. 40.9.+-. 40.4.+-.
40.4.+-. 2.2 1.1 0.8 0.7 58 47.2.+-. 41.9.+-. 41.3.+-. 41.5.+-. 2.0
0.7 0.9 0.8 59 48.5.+-. 43.0.+-. 42.3.+-. 42.3.+-. 2.9 1.0 0.5 0.5
______________________________________
TABLE V ______________________________________ AVERAGE ENERGY AND
ENERGY SPREAD IN MeV AS A FUNCTION OF THE NUMBER AND ORIGIN OF 5%
FIELD STEPS Origin of Number of 5% Steps Perturbations 5 10 15 20
______________________________________ 50 36.5.+-. 33.3.+-.
33.0.+-. 33.2.+-. 1.2 0.7 0.5 0.6 51 37.1.+-. 34.0.+-. 33.8.+-.
33.9.+-. 1.1 0.5 0.5 0.5 52 38.4.+-. 34.9.+-. 34.8.+-. 34.9.+-. 1.4
0.7 0.8 0.7 53 39.0.+-. 35.7.+-. 35.6.+-. 35.7.+-. 1.4 1.2 0.5 0.4
54 40.0.+-. 36.8.+-. 36.5.+-. 36.7.+-. 1.4 0.8 0.7 0.7 55 41.0.+-.
37.7.+-. 37.4.+-. 37.5.+-. 1.4 0.6 0.4 0.5 56 42.1.+-. 38.5.+-.
38.1.+-. 38.4.+-. 1.5 0.9 0.6 0.6 57 43.3.+-. 39.4.+-. 39.1.+-.
39.3.+-. 1.6 0.6 0.5 0.5 58 43.8.+-. 40.3.+-. 39.9.+-. 40.1.+-. 1.5
0.7 0.4 0.5 59 44.9.+-. 41.3.+-. 40.9.+-. 41.1.+-. 1.4 0.9 0.7 0.8
______________________________________
Higher energies result when the perturbations are moved toward the
high-energy end of the drift-tube linear accelerator 11 and lower
energies result when the perturbations are moved toward the
low-energy end of the drift-tube linear accelerator 11.
In a permanent-magnet focused type of drift-tube linear accelerator
11, a lower limit to the energies exists for which the present
invention is suitable and below which the particle beam becomes
unstable. In a 70-MeV drift-tube linear accelerator, for example,
this limit is about 20 MeV.
Table I shows that five 2% perturbations give a field reduction
factor of only 0.939, which is not low enough to drop the particle
beam out of synchronization. The left-hand column of Table II
confirms that situation, showing the average energy in each case to
be close to unperturbed value of 70 MeV. All other combinations in
Table I show an energy reduction capability. However, those
combinations with field reduction factors exceeding 0.8 yield the
largest energy spreads in Tables II-VI. Ten 4% perturbations give a
field reduction factor of 0.6648 which will yield a relatively
well-defined energy-reduction capability with root-mean-square
energy spreads of 1 MeV or less.
In order to achieve all of the field distributions illustrated in
FIG. 3, a proportional control of the magnitude of the
perturbations on the individual post couplers 15 would be required.
However, in accord with the present invention, field distribution
can be controlled with simple binary control of the number and
location of the post couplers 15 producing perturbations of fixed
magnitude, see FIGS. 4 and 5. As shown, the present invention can
yield any desired energy, within the limits of the drift-tube
linear accelerator 11, to a resolution of 1 MeV or less and an
energy spread of .+-.1 MeV or less.
With reference again to FIG. 1, it can be appreciated that each
post coupler 15 can be set to one of two positions via a binary
control signal from post-coupler controller 25. Thus by programming
of the post coupler controller 25 a great selection of energy
levels can be easily achieved and readily modified. Depending upon
particular application requirements the post-coupler controller can
be physically realized by virtually anything from a set of manually
operated switches to a digital decoder responsive to programmed
control information.
The foregoing description of a preferred embodiment of the
invention has been presented for purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise form disclosed, and obviously many
modifications and variations are possible in light of the above
teaching. The embodiment was chosen and described in order to best
explain the principles of the invention and its practical
application to thereby enable others skilled in the art to best
utilize the invention in various embodiments and with various
modifications as are suited to the particular use contemplated. It
is intended that the scope of the invention be defined by the
claims appended hereto.
* * * * *